The present invention relates to a bicycle frame and a bicycle including the bicycle frame. More particularly, but not exclusively, it relates to a frame for a bicycle with an improved suspension system.
The invention is applicable to mountain bicycles; cross country bicycles; racing or touring bicycles. It is however, particularly relevant to mountain bikes and will be described herein in that context.
Mountain bicycles were developed for riding across rough and steep terrain. The original designs featured rigid frames having configurations similar to those of conventional touring bicycles. The only means available to insulate the bicycle's rider and structure from shocks when crossing rough terrain was the use of large pneumatic tyres. This could lead to rapid rider fatigue, and required robust construction or expensive materials of construction to reduce the risk of damage to the bicycle.
It is known to provide shock absorbers, incorporated into the front forks of a bicycle, to act as a suspension for the front wheel. This is of some benefit, but since the majority of the rider's weight is usually supported by the rear wheel, front wheel suspension is at best only a partial solution.
Hence, there have been attempts to provide so-called “full-suspension” frames, of which there are several variations.
As a general principle, they comprise a secondary frame on which the rear wheel is mounted, and which is pivoted to the main frame of the bicycle at or near the “bottom bracket”, the point at which the pedals are mounted to the frame. This secondary frame, or “swingarm”, is also connected to the main frame via a shock absorber. Impacts on the rear wheel from rough terrain cause the rear wheel and swingarm to pivot as a unit, the shock absorber damping out the worst of the impacts.
However, this general arrangement does have significant shortcomings. The sensitivity and effectiveness of the system can be critically dependent on the exact configuration of the pivot point relative to the bottom bracket. Sideways forces exerted on the rear wheel or on the frame can be transmitted through the shock absorber without being damped, and can tend to twist components out of alignment and may tend to loosen or damage vital joints, especially the pivot point between the swing arm and the mainframe. The swingarm and rear wheel are constrained to move in an arc of a circle, which may not be an optimum response to an impact or shock. Furthermore, a full-suspension frame is significantly heavier and more complicated to construct and maintain than a conventional frame.
In a “linkage” type of full-suspension frame, the swing-arm is not a single unit but comprises three linkages, each one pivoted to the next in series, and pivoted at each end to the main frame of the bicycle. One of these linkages is also connected to the main frame via a shock absorber. This allows a more linear motion of the rear wheel under impact, since it is mounted to the linkage not pivoted directly to the main frame. However, it can be even heavier than the normal full-suspension frame, it is more complex to build and maintain, and the number of individual pivots and linkages greatly increase the chances of misalignment and bearing failure.
In the “URT” (unified rear triangle) variant of a full-suspension frame, a triangular swingarm, bearing both the rear wheel mounting and the bottom bracket, pivots on the main frame and is also connected to the main frame via a shock absorber, but the distance between the rider's seat and the pedals can change during the action of the suspension. The weight distribution can be unpredictable, especially should the rider change from sitting on the saddle to standing up on the pedals. In this latter case, he is no longer insulated from impacts on the rear wheel.
The “flex” system relies on the “chain stay”, the section of a conventionally configured frame which connects the bottom bracket to the point at which the rear wheel is attached comprising an inherently flexible material. A shock absorbing element is present in the seat stays, the section of a conventionally configured frame which connects the point at which the rear wheel is attached to the top of the frame, adjacent the point of attachment of the rider's seat. Such a frame is relatively lightweight and simple, but it does not provide as much damping as the other forms of suspension, and it tends to be flexible laterally as well as vertically, leading to severe misalignment problems.